w1 bulk read per master

[herby74]

A bulk read of slave devices of a w1_bus_master should be possible by writing trigger to the therm_bulk_read entry in the device tree of the bus master. However, the entry therm_bulk_read only shows up for the first w1_bus_master. (Reason: The static variable counter u16 bulk_read_device_counter is common for all masters in w1_therm.c.) This change fixes the behavior by using a dedicated bulk_slave_count variable per master.